You MUST MUST MUST do the Manta night snorkel in Kona. We did it with Manta Ray Dives of Hawaii and it was the most fantastic thing I have ever done! They also offer an Eco Tour which looks great (whale sharks, regular sharks, seals and such) but i did not see that until I got home. Also swimming with the dolphins was awesome, although I was the only one in the water at the time which made it that much better. That was done with Kona Adventures.
We were there last month and did a day tour with Mary Lou's Big Island Tours in Hilo. They were great, taking us to Volcano National Park and Rainbow Falls. We walked in a lava tube/cave at Rainbow Falls which was interesting. Something that can not be done here in Florida, LOL. There is no more flowing lava in Hawaii, but the steam vents were going and you can see the huge crater the eruption left. Well worth the price of admission to the park. They also do day tours in Kona too. Very good if you are into that sort of thing.
The book Hawaii The Big island Revealed is great to take in your car and follow his advice. We drove around the island stopping at all the places suggested in the book. Easy to follow and gave the history of the places, great directions and even bathroom stops along the way. In Hilo, do the scenic route off the highway...it is really superb. If you see cars stopped along the side of the road, be adventurous and stop and follow the trails. We found some beautiful spots along the scenic route that way.
I will think more about it and post again if I remember anything else we loved. OH...shaved ice with a snow cap!
